1. Field of the Invention
This invention relates to eddy current probes, and more specifically to a probe with at least one solenoid coil generating a electromagnetic field, the field focused and spatially discretized through an array of posts through which a magnetic field passes.
2. Prior Art
It is known in the art that variations in conductivity and permeability of a material indicate the presence of structural defects such as cracks and corrosion. These variations can be measured by propagating a primary magnetic field into the material to create eddy currents. The eddy currents generated in the material then generate a return magnetic field that is detected by the probe coil. Defects in such materials that decrease the conductivity and disrupt the eddy currents cause the magnitude of the return magnet field to decrease.
When the material is without flaws, the two magnetic fields are largely out of phase and the fields partially cancel, which reduces the coil voltage. Therefore, the probe coil voltage increases to indicate that the test coil is adjacent a defect. Signature characteristics of the flaw appear as a small modulation of the return magnetic field carrier signal. Thus, the sensitivity of the probe and the ability to sense the signature of the flaw is directly dependent on the magnitude of the incident primary magnetic field. It is therefore advantageous to have a magnetic field maximum field strength.
An eddy current probe comprises at least one solenoid generating a magnetic field. The field passes through a directed field array of mutually spaced apart high permeability posts that discretizes the magnetic field into separate magnetic fields with increased field strength. The field passes from the solenoid center, typically including a core, into an array base and through the posts and not through air space separating the posts. The magnetic field is thereby focused into the respective posts resulting in increased field strength. Fields of increased strength then emerge from post ends in a pattern of discretized and regularly-separated magnetic fields. Such a pattern of closely arranged but separated magnetic fields are particularly advantageous in eddy current signal processing. The discretized magnetic fields generate eddy currents at a well-defined higher spatial resolution much improved over the resolution that an unimproved same-sized solenoid would otherwise produce. A pick-up coil at or near each post end is positioned to detect small features within the material in coordination with the same resolution directed by the several posts. Thus a single large core can be used to generate a plurality of discrete magnetic fields with fine resolution that are disposed to excite eddy currents in a proximate test material. The eddy currents generate return magnetic fields that are detected at the point of excitation by the pick-up coil at the post end. Use of the single large core as the field-generating source is more cost-effective in probe production and more reliable in performance than a plurality of much smaller solenoid coils that would have to substitute for the several posts for similar functional effect.
A first embodiment of the eddy current probe directed field array includes a planar base installed transverse a solenoid coil at the end of a coil center or core. The array of posts extends from the base away from the solenoid longitudinally with the solenoid axis. The magnetic field generated by the solenoid along the solenoid axis then propagates through the posts in enhanced and discretized magnetic fields divided from the original solenoid magnetic field. Such a configuration is amenable for testing of flat or near flat surfaces, such as an airplane wing, by passing the array of posts over the flat surface with the posts perpendicular to the surface.
A second embodiment of the directed field array is an array of posts extending from the base radially, spaced apart circumferentially about the base. The magnetic field is then directed through the base and through the array of posts radially from the solenoid axis. Such a configuration is amenable for testing an inner surface of a tubular material by passing the solenoid through the material, the post ends extending to a position proximate the tube inner surface.
An enhancement of this second embodiment is to configure two such cylindrical solenoids axially end to end. The base may lie transversely across and between solenoid ends or the base may be annular around a single core common to coils of both solenoids, lying intermediate the solenoid core, typically central. Opposing magnetic fields combine and pass through the annular base, or ring, radially from the solenoid. (For these purposes, xe2x80x9copposing magnetic fieldsxe2x80x9d or similar term means fields of like polarity that mutually repel.) The combined electromagnetic fields of the two opposing solenoids are thus focused through and out of the ends of the posts of the central base.
End bases may be located at distal ends of the end-to-end solenoids transverse the solenoid axis. The end bases may also include radial posts disposed outward about base circumferences. Magnetic fields effectively emerge from the posts of the central base with a large radial component and return to the posts of the end bases. This configuration of a pair of solenoids bounded by radial rings produces a combined magnetic field greater than the field from either solenoid alone. The enhanced and discretized magnetic field is thus better able to penetrate a test material proximate the post ends and is especially suitable as a probe to excite eddy currents for measuring and locating defects from within a tube as the probe is advanced through the tube.
The combined magnetic field emanating from the flat ends of the posts divide into symmetric fields about the ring each adapted to penetrate into a tubular test material proximate the rings. The combined magnetic field comprises a lead field symmetric with a following field that is concentrated radially outward from the center ring. With two symmetric and identical fields, as the dual solenoid configuration moves axially in a tube, one of the magnetic fields, as a lead field, is disposed to first encounter an anomaly in the tube before the other, or following magnetic field encounters it. Return magnetic fields generated by eddy currents in the tube that are induced by the lead and following magnetic fields are thus amenable to differencing that removes a carrier component common in the two return magnetic fields largely leaving only the component due to the material anomaly that is modulated on the carrier component.